Fracture and mechanical stratigraphy at meter-mm scale of the Whitby Mudstone Formation_

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Fracture and mechanical stratigraphy at meter-mm scale of the Whitby Mudstone Formation_

Dik, R.

(2016) Faculty of Geosciences Theses

(Master thesis)

Abstract

Production of gas is becoming increasingly challenging. New prospects and methods are required in order for the Netherlands to stay self-sustaining, as the golden age for Dutch gas is coming to an end. Conventional gas reserves are dwindling, and this leads towards the possibility of unconventional resources, with shale gas ... read more in particular. Production of shale gas in Europe is ambitious, due to the extremely low permeability of the prospects. Natural fractures can play an important role in the production of hydrocarbons from unconventional plays. Studying these features is essential in order to get a better understanding of rock behaviour during hydraulic stimulation of tight gas reservoirs, as natural fractures can greatly increase permeability of a rock and also affect the mechanics of hydraulic stimulation.
The aim of this research is to find out whether or not the Posidonia Shale Formation, the main Dutch shale gas prospect, is a decent prospect. This will be done by analysis of fracture and mechanical stratigraphy of the Whitby Mudstone Formation, the lateral time equivalent of the Posidonia Shale Formation. The Whitby Mudstone Formation is characterized by a set of sub-vertical joints with two main strike directions (N-S and E-W). Vertical fractures were studied and measured from cliff sections in Yorkshire (UK). These fractures were then and digitized from orthophotos, created from photographs taken from the cliff side. The orthophotos were created in two main directions, parallel to the two different fracture sets. This data is interpreted in terms of fracture length, fracture density as well as critical spacing. Samples from the field were analysed for their mineralogical composition throughout the stratigraphy.
Results indicate that the length and amount of fractures differ per bed. The amount of fractures, where the fractures are located and on what bedding interfaces the fractures terminate is related to rock composition and brittleness index in order to provide a meaningful insight. An example is that thinner and more quartz rich layers contain a higher number of fractures than layers with lower quartz content. Variations in composition are usually minor, as the samples consist mainly of clay minerals (up to 75%).
The collected data adds indirectly to the understanding of the Posidonia Shale Formation. Results point out that the Whitby Mudstone Formation is indeed a potential outcrop equivalent, as mineral composition (most importantly TOC content of up to 15%), porosity and permeability are similar to that of the Posidonia Shale Formation. However, calculated outcomes for the brittleness index (up to 0.2) are low. To obtain a well-balanced opinion on the Whitby Mudstone Formation and Posidonia Shale Formation more subsurface information is required, as exhumation and weathering processes might have influenced the rocks and structures. show less

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